Optical disk devices are used for the storage of computer-prepared data and have recognized value in their ability to store large quantities of data. The media for use in such devices is reactive to the intensity modulation of light, such as may be produced by the rapid switching of a semi-conductor laser. In order to write data on optical media, the laser power must be controlled at a relatively high power level, in order that the media can be altered in accordance with the input data stream. In reading the data back, the laser power level is controlled at a lower level so that the media is not altered by the laser beam, but the reflected light indicates the presence or absence of media alterations caused by the input data stream.
In operating an optical disk system, it is necessary to set the correct laser power level to read and to write for each optical disk. The correct parameters for the optical disk are included in information in an identification header stamped onto the disk itself. That information, when read by the system, enables a calibration circuit to set the desired current levels for the laser to produce the correct laser power. Since, however, the laser is subject to unintended changes in its operating parameters, particularly with temperature and aging, the calibration method is also used to change current levels for the laser so that the expected power level is maintained under operating conditions and throughout laser life.
The common practice of calibrating laser circuits to operate with a given optical medium usually involves producing the desired laser light intensity at the target (optical medium). To do that, laser control circuits are set to produce the predetermined or desired light intensity at the optical medium. Analysis is conducted to determine digital-to-analog converter (DAC) settings to produce the laser power needed in the writing and the erasing operations. Additionally, read power levels are established.
The initial analysis is conducted on the manufacturing line so that correct values are established within the control system. These initially established values that include the relationship between power sensed at a photodetector to the total power produced by the laser to gain an accurate measure of the power incident on the target (optical medium). Thereafter, during operation, changes in power sensed by the photodetector cause the control circuit to alter the current driving the laser in order to bring power at the photodetector back to the desired level. In that manner, it is believed that power at the optical disk will be produced at the correct value.
In order to provide a portion of the power produced by the laser to the photodetector, a beamsplitter is placed in the optical path to divert a small portion of the power to the photodetector. As long as the power received at the photodetector accurately reflects changes in the power produced by the laser, the circuit works properly. However, should the power at the photodetector change independently of power produced by the laser there would be an alteration of the current driving the laser independent of the objectives of the control system. It has been found that such a problem does occur when the effects of temperature and humidity, that is ambient operating conditions, cause a change in the power received by the photodetector which is independent of changes at the laser source. Such independent changes are largely due to changes in the transmissivity and the reflectivity of the beamsplitter. Attempts have been made to produce a prism whose reflective coating does not change with temperature and humidity but the small tolerances required result in very expensive parts. The inventors herein have taken a different approach and have produced a system that is relatively free of changes in transmissivity and reflectivity within the optical system while maintaining the cost of the system at a reasonable level.